Fischer-Tropsch synthesis process

ABSTRACT

A process for the conversion of synthesis gas to higher hydrocarbons by synthesis gas, at an elevated temperature and pressure, with a suspension of a particulate Fischer-Tropsch catalyst, in a system comprising at least one high shear mixing zone and a reactor vessel wherein the process comprises: (a) passing the suspension and the gaseous stream through the high shear mixing zone wherein the gaseous stream is broken down into gas bubbles and/or irregularly shaped gas voids; (b) discharging suspension having gas bubbles and/or irregularly shaped gas voids dispersed therein from the high shear mixing zone into the reactor vessel; and (c) maintaining the temperature of the suspension discharged into the reactor vessel at the desired reaction temperature by means of an internal heat exchanger positioned within the suspension in the reactor vessel. At least 5% of the exothermic heat of reaction is removed from the system by means of the internal heat exchanger. The remainder of the exothermic heat of reaction may be removed from the system by means of an external heat exchanger and/or through the introduction of a liquid coolant.

This application is the U.S. National Phase of International ApplicationPCT/GB02/02307, filed May 17, 2002, which designated the U.S.

BACKGROUND OF THE INVENTION

The present invention relates to a process for the conversion of carbonmonoxide and hydrogen (synthesis gas) to liquid hydrocarbon products inthe presence of a Fischer-Tropsch catalyst.

In the Fischer-Tropsch synthesis reaction a gaseous mixture of carbonmonoxide and hydrogen is reacted in the presence of a catalyst to give ahydrocarbon mixture having a relatively broad molecular weightdistribution. This product is predominantly straight chain, saturatedhydrocarbons which typically have a chain length of more than 2 carbonatoms, for example, more than 5 carbon atoms. The reaction is highlyexothermic and therefore heat removal is one of the primary constraintsof all Fischer-Tropsch processes. This has directed commercial processesaway from fixed bed operation to slurry systems. Such slurry systemsemploy a suspension of catalyst particles in a liquid medium therebyallowing both the gross temperature control and the local temperaturecontrol (in the vicinity of individual catalyst particles) to besignificantly improved compared with fixed bed operation.

Fischer-Tropsch processes are known which employ slurry bubble columnsin which the catalyst is primarily distributed and suspended in theslurry by the energy imparted from the synthesis gas rising from the gasdistribution means at the bottom of the slurry bubble column asdescribed in, for example, U.S. Pat. No. 5,252,613.

The Fischer-Tropsch process may also be operated by passing a stream ofthe liquid medium through a catalyst bed to support and disperse thecatalyst, as described in U.S. Pat. No. 5,776,988. In this approach thecatalyst is more uniformly dispersed throughout the liquid mediumallowing improvements in the operability and productivity of the processto be obtained.

GB 728543 relates to a process for the production of hydrocarbons by thereaction of synthesis gas in the presence of a catalyst which may besuspended in finely divided form within the hydrocarbon oil (contactoil). A mechanically moved stream of contact oil circulating after theseparation of the gas, and the synthesis gas is introduced into thereaction chamber below a cooling arrangement disposed therein, suitablyby means of one or a series of nozzles. Cooling of the contact oil ormixture of contact oil and gas in the reaction chamber is effected in anumber of stages in such manner that the mixture of synthesis gas andcontact oil successively flows through cooling stages at increasingtemperature. Owing to the fact that the individual cooling stages have atemperature increasing from the bottom upwards, the reaction can beretarded in places where the concentration of carbon monoxide andhydrogen is highest, namely in the lower part of the reaction tower, bythe application of low temperatures. In accordance with the reduction ofthe concentration of the reaction substances, the temperature is thenincreased in the higher zones of the reaction tower, so that thecomplete reaction between the carbon monoxide and the hydrogen,corresponding substantially to equilibrium, is obtained in theneighborhood of the top of the reaction tower. Thus, GB 728,543 relatesto a plug flow reactor vessel where the reaction conditions vary in theindividual cooling stages.

SUMMARY OF THE INVENTION

We have recently found that a Fischer-Tropsch process may be operated bycontacting synthesis gas with a suspension of catalyst in a liquidmedium in a system comprising at least one high shear mixing zone and areactor vessel. The suspension is passed through the high shear mixingzone(s) where synthesis gas is mixed with the suspension underconditions of high shear. The shearing forces exerted on the suspensionin the high shear mixing zone(s) are sufficiently high that thesynthesis gas is broken down into gas bubbles and/or irregularly shapedgas voids. Suspension having gas bubbles and/or irregularly shaped gasvoids dispersed therein is discharged from the high shear mixing zone(s)into the reactor vessel where mixing is aided through the action of thegas bubbles and/or the irregularly shaped gas voids on the suspension.The suspension present in the reactor vessel is under such highlyturbulent motion that any irregularly shaped gas voids are constantlycoalescing and fragmenting on a rapid time scale, for example over atime frame of up to 500 milliseconds, typically between 10 and 500milliseconds. The transient nature of these irregularly shaped gas voidsresults in improved heat transfer and mass transfer of gas into theliquid phase of the suspension when compared with a conventional slurrybubble column reactor. The reactor system may therefore be regarded as acontinuous stirred tank reactor (CSTR) with the conditions oftemperature and pressure and the concentration of reactants and productsbeing substantially constant throughout the body of suspension in thereactor vessel. The reactor vessel may be a tank reactor in which case asuspension recycle stream is withdrawn from the reactor vessel and maybe recycled to the high shear mixing zone(s) via an external conduit.Exothermic heat of reaction may be removed from the system by means of aheat exchanger positioned in the external conduit (external heatexchanger). Optionally, further exothermic heat of reaction may beremoved from the system by means of a heat exchanger, for example,cooling tubes or coils positioned within the suspension in the reactorvessel (internal heat exchanger). This process is described in WO0138269 (PCT patent application number GB 0004444) which is hereinincorporated by reference. However, a problem may arise when operatingthe process of WO 0138269 in that there maybe a limit on the temperatureto which the suspension may be cooled by the external heat exchangerowing to the risk of quenching the reaction and/or deactivating thecatalyst. In the absence of an internal heat exchanger, this maynecessitate circulating suspension around the external loop conduit atan uneconomic flow rate.

It has now been found that where a slurry process is operated in areactor system comprising at least one high shear mixing zone and areactor vessel that it is advantageous to remove at least a 5% of theexothermic heat of reaction from the system by means of an internal heatexchanger.

Accordingly, the present invention relates to a process for theconversion of synthesis gas to higher hydrocarbons by contacting agaseous stream comprising synthesis gas, at an elevated temperature andpressure, with a suspension comprising a particulate Fischer-Tropschcatalyst suspended in a liquid medium, in a system comprising at leastone high shear mixing zone and a reactor vessel wherein the processcomprises:

-   (a) passing the suspension and the gaseous stream through the high    shear mixing zone(s) wherein the gaseous stream is broken down into    gas bubbles and/or irregularly shaped gas voids;-   (b) discharging suspension having gas bubbles and/or irregularly    shaped gas voids dispersed therein from the high shear mixing    zone(s) into the reactor vessel;-   (c) maintaining the temperature of the suspension discharged into    the reactor vessel at the desired reaction temperature by means of    an internal heat exchanger positioned within the suspension in the    reactor vessel characterized in that at least 5% of the exothermic    heat of reaction is removed from the system by means of the internal    heat exchanger.

For avoidance of doubt, conversion of synthesis gas to higherhydrocarbons may be initiated in the high shear mixing zone(s). However,it is envisaged that the majority of the conversion of the synthesis gasto higher hydrocarbons will take place in the reactor vessel.

Typically, at least 10%, preferably at least 20%, more preferably atleast 40%, most preferably at least 50% of the exothermic heat ofreaction is removed from the system by means of the internal heatexchanger. It is envisaged that substantially all of the exothermic heatof reaction may be removed from the system by means of the internal heatexchanger. However, it is preferred that between 20 to 50%, morepreferably 30 to 50% of the exothermic heat of reaction is removed fromthe system by means of the internal heat exchanger.

Preferably, suspension is withdrawn from the reactor vessel and is atleast in part recycled to the high shear mixing zone(s) (hereinafterreferred to as “suspension recycle stream”). Suitably, the suspensionrecycle stream is cooled outside of the reactor vessel and high shearmixing zone(s) by means of a further heat exchanger (hereinafter“external heat exchanger”) in order to further assist in the removal ofexothermic heat of reaction from the system. Preferably, between 20 to55%, more preferably, 30 to 50%, for example 40 to 50% of the exothermicheat of reaction is removed from the system in the external heatexchanger. Preferably, the suspension recycle stream is cooled to atemperature not more than 30° C. below, preferably, not more 12° C.below, the temperature of the suspension in the reactor vessel.

Preferably, suspension is withdrawn from the reactor vessel and isrecycled to the high shear mixing zone(s) by means of an externalconduit having a first end in communication with an outlet (for thesuspension) in the reactor vessel and a second end in communication withthe high shear mixing zone(s). Suitably, the external heat exchanger maybe positioned on the external conduit. Preferably, the ratio of thevolume of the external conduit (excluding the volume of the externalheat exchanger) to the volume of the reactor vessel is in the range of0.005:1 to 0.2:1. Suitably, a mechanical pumping means, for example, aslurry pump is positioned in the external conduit, preferably upstreamof the external heat exchanger.

For a 30,000 barrel/day commercial scale plant, the suspension issuitably recycled through the external conduit at a rate of between10,000 to 50,000 m³/h, preferably, 15,000 to 30,000 m³/h, morepreferably 17,000 to 25,000 m³/h. For a 60,000 barrel/day commercialplant, the suspension is recycled through the external conduit at a rateof between 20,000 to 100,000 m³/h. Thus, the rate at which thesuspension is recycled through the external conduit will be pro rata tothe size of the commercial scale plant. Suitably, the flow rate throughthe external conduit may be in the range (n×10,000) m³/h to (n×50,000)m³/h for a (n×30,000) barrel/day commercial plant where n is a number inthe range 0.25 to 10, preferably 1.5 to 5.

Preferably, up to 50% by volume, more preferably up to 20% by volume ofthe hydrogen component of the synthesis gas (hereinafter “hydrogencomponent”) is introduced into the suspension recycle stream.

Without wishing to be bound by any theory, it is believed that theunconverted synthesis gas which is present in the suspension recyclestream may be depleted in hydrogen. An advantage of injecting thehydrogen component into the suspension recycle stream is that this willmaintain the ratio of hydrogen to carbon monoxide in the synthesis gasat an optimum value thereby improving the conversion of synthesis gas tohigher hydrocarbons. A further advantage of injecting the hydrogencomponent into the suspension recycle stream is that this may alsostabilize the catalyst.

It is also envisaged that up to 50% by volume, preferably up to 20% byvolume of the carbon monoxide component of the synthesis gas(hereinafter “carbon monoxide component”) may be introduced into thesuspension recycle stream.

Suitably, the hydrogen component and/or the carbon monoxide component isintroduced into the external conduit either upstream or downstream ofthe mechanical pumping means, preferably downstream of the mechanicalpumping means. Preferably, the hydrogen component and/or carbon monoxidecomponent is introduced to the external conduit upstream of the externalheat exchanger. The hydrogen component and/or the carbon monoxidecomponent may be introduced into the external conduit at more than oneposition along the length of the external conduit.

Preferably, the hydrogen component and/or the carbon monoxide componentis introduced into the external conduit via a gas nozzle. Preferably,the pressure drop over the gas nozzle is at least 0.1 bar, morepreferably, at least 0.5 bar, for example, at least 1 bar.

Where necessary, the ratio of hydrogen to carbon monoxide in theunconverted synthesis gas within the reactor vessel may be adjusted byfeeding additional hydrogen and/or carbon monoxide directly into thereactor vessel, for example, via a gas sparger.

Where the hydrogen component is introduced into the suspension recyclestream in the substantial absence of carbon monoxide, the hydrogen maybe obtained from synthesis gas, for example, the hydrogen may beseparated from synthesis gas by pressure swing adsorption or bydiffusion through a membrane system.

Suitably, the reactor vessel has a diameter of from 5 to 15 meters,preferably 7.5 to 10 meters, more preferably 7.5 to 8 meters. Suitably,the reactor vessel has a length of from 5 to 30 meters, preferably 10 to20 meters, for example, 15 to 20 meters. For practical reasons, thereactor vessel may be operated with a headspace. Where the reactorvessel has a headspace, the height of the suspension in the reactorvessel is preferably at least 7.5 meters, preferably at least 10 meters.

Preferably, the reactor vessel approximates to a continuous stirred tankreactor (CSTR) having a Peclet number of less than 3, more preferablyless than 1, even more preferably approaching zero. The Peclet (Pe)number is defined by the equation:Pe=U _(g) H/δwhere U_(g) is the gas velocity (ms⁻¹), H is the height of thesuspension in the reactor vessel (m), δ is the dispersion coefficient(m²s⁻¹) (see Carberry, J. J., ‘Chemical and Catalytic ReactionEngineering’, McGraw-Hill, 1976, or Levenspiel, O., ‘Chemical ReactionEngineering’, Wiley, 1972.).

Owing to the well mixed nature of the suspension in the reactor vessel,it is possible to operate the internal heat exchanger with a largetemperature difference between the coolant liquid which is fed to theheat exchanger (hereinafter referred to as “heat exchange liquid”) andthe temperature of the suspension in the reactor vessel, without anyrisk of quenching the Fischer-Tropsch synthesis reaction. Thus, the heatexchange liquid fed to the internal heat exchanger is preferably at atemperature which is at least 12° C. below, more preferably, at least25° C. below, most preferably at least 50° C. below, for example, atleast 100° C. below the temperature of the suspension in the reactorvessel. Typically, the heat exchange liquid is fed to the internal heatexchanger at a temperature of less than 100° C., preferably less than50° C. A suitable heat exchange liquid is water, a solution of aninorganic salt, molten inorganic salts, a high boiling point oil, aglycol or liquid sodium, preferably water.

Preferably, the internal heat exchanger comprises an array of coolingtubes and/or cooling coils and/or cooling plates. Suitably, the arraymay be divided into independently operated banks of cooling tubes and/orcooling coils and/or cooling plates (hereinafter “banks”). Preferably,the array comprises 5 to 500, more preferably 50 to 500, most preferably100 to 500 such banks. Suitably, a bank comprises 5 to 50 cooling tubes,or 5 to 20 cooling coils or 5 to 20 cooling plates. The amount of heatwhich may be removed from the system using the array may be adjusted by(a) independently adjusting the temperature of the heat exchange liquidwhich is fed to the banks and/or (b) increasing or decreasing the numberof banks to which the heat exchange fluid is fed. However, it ispreferred to supply heat exchange fluid to all of the banks of thearray. Suitably, the temperature of the heat exchange fluid which is fedto at least some of the banks is at least 12° C. below, preferably atleast 25° C. below, more preferably at least 50° C. below, for example,at least 100° C. below the temperature of the suspension in the reactorvessel.

Where the heat exchanger comprises an array of cooling tubes, it ispreferred that the cooling tubes are arranged substantially parallel toone another with the longitudinal axes of the cooling tubes aligned withthe longitudinal axis of the reactor vessel. Preferably, the coolingtubes have an outer diameter in the range 0.625 to 15 cm, morepreferably 1.25 to 7.5 cm, most preferably 2 to 5 cm, for example 2.5cm. Preferably, the cooling tubes may be finned so as to provide agreater heat transfer surface area within the reactor vessel. Suitably,the cooling tubes lie below the level of suspension in the reactorvessel and preferably extend through substantially the whole of theheight of the suspension in the reactor vessel, preferably through up to80% of the height of the suspension in the reactor, more preferablythrough up to 60% of the height of the suspension in the reactor vessel.Preferably, the cooling tubes are spaced from each other or from thewalls of the reactor by 5 to 60 cm, preferably, 7.5 to 25 cm, morepreferably 10 to 20 cm, for example, 12.5 cm. Suitably, cooling tubesare absent from the “blast” or “injection” zone(s) of the high shearmixing zone(s) i.e. the region of the reactor vessel into which the highshear mixing zone(s) discharges its contents. Where the high shearmixing zone(s) has a circular outlet, the “blast” or “injection” zonelies within a cylindrical region of the reactor vessel. The centre ofthe circular section of this cylindrical region is aligned with thecentre of the outlet of the high shear mixing zone. Suitably, thediameter of the circular section of the cylindrical region is at least 2times, preferably at least 3 times the diameter of the outlet of thehigh shear mixing zone.

Where the heat exchanger comprises an array of cooling coils, eachcooling coil may be in the form of a helix with the coil wound as ifalong a cylinder (hereinafter “cylinder defined by the helix”). It ispreferred that the “blast” or “injection” zone of a high shear mixingzone lies within the cylinder defined by the helix. Suitably, thediameter of the cylinder defined by the helix is at least 2 times thediameter of the outlet of a high shear mixing zone, preferably at least3 times. Preferably, the tubing of the cooling coils has an outerdiameter of between 2.5 cm and 10 cm. Preferably, the cooling coils arefinned so as to provide a greater heat transfer surface area. Suitably,the cooling coils are spaced apart from each other or from the walls ofthe reactor vessel by 5 to 60 cm, preferably, 7.5 to 25 cm, morepreferably 10 to 20 cm, for example, 12.5 cm. Suitably, the coolingcoils lie below the level of suspension in the reactor vessel asdescribed above for the cooling tubes.

Where the heat exchanger comprises an array of cooling plates, it ispreferred that cooling plates are concertinaed or corrugated so as toincrease the heat transfer area. Preferably, the cooling plates have abreadth of 2 to 10 cm. Preferably, the cooling plates have a depth(distance across the folds of the concertinaed plates or between thepeaks and troughs of the corrugated plates) of 10 to 50 cm. Preferably,the cooling plates are spaced apart from each other and from the wallsof the reactor vessel by at least 10 cm. Preferably, the longitudinalaxes of the cooling plates are aligned with the longitudinal axis of thereactor vessel. Suitably, where the cooling plates are arranged withtheir longitudinal axes aligned with the vertical axis of the reactorvessel, the cooling plates have a length of 60 to 80% of the height ofthe suspension in the reactor vessel.

Further cooling may also be provided to the system by introducing aliquid coolant to the reactor vessel and/or the high shear mixingzone(s) and/or to the external conduit. Preferably, the introduction ofthe liquid coolant removes at least 5%, preferably at least 10% of theexothermic heat of reaction from the system. The liquid coolant may beany liquid which is compatible with a Fischer-Tropsch synthesisreaction. Preferably, the liquid coolant which is to be introduced intothe system is at a temperature which is substantially below thetemperature of the suspension in the reactor vessel. Preferably, theliquid coolant is at a temperature which is at least 25° C. below,preferably at least 50° C. below, more preferably at least 100° C. belowthe temperature of the suspension in the reactor vessel. Suitably, theliquid coolant is cooled (e.g. using refrigeration techniques) beforebeing introduced into the system. Preferably, the liquid coolant iscooled to a temperature below 15° C., more preferably, below 10° C.

Preferably, the liquid coolant is a solvent which is capable ofvaporizing under the process conditions (i.e. at an elevated temperatureand pressure). Such a liquid coolant is hereinafter referred to as“vaporizable liquid coolant”). Without wishing to be bound by any theoryit is believed that the latent heat of vaporization of the vaporizableliquid coolant removes at least some of the exothermic heat of reactionfrom the system.

Suitably, the vaporizable liquid coolant has a boiling point, atstandard pressure, in the range of from 30 to 280° C., preferably from30 to 100° C. Preferably, the vaporizable liquid coolant is selectedfrom the group consisting of aliphatic hydrocarbons having from 5 to 10carbon atoms, alcohols (preferably, alcohols having from 1 to 4 carbonatoms, in particular, methanol and ethanol or a glycol), ethers (forexample, dimethyl ether) tetrahydrofuran, and water. In order tosimplify the process, it is preferred that the vaporizable liquidcoolant is selected from the group consisting of water (a by-product ofthe Fischer-Tropsch synthesis reaction) and low boiling liquid higherhydrocarbons, such as higher hydrocarbon having from 5 to 10 carbonatoms, in particular, pentanes, hexanes, or hexenes.

The high shear mixing zone(s) may be part of the system inside oroutside the reactor vessel, for example, the high shear mixing zone(s)may project through the walls of the reactor vessel such that the highshear mixing zone(s) discharges its contents into the reactor vessel.Preferably, the reactor system comprises a plurality of high shearmixing zones. Preferably the reactor system comprises 2 to 500 highshear mixing zones, more preferably 10 to 400 high shear mixing zones,most preferably, 20 to 300, for example, 25 to 150 high shear mixingzones. The high shear mixing zones may discharge into a single reactorvessel which has an advantage of significantly reducing the size of acommercial plant. However, it is also envisaged that the high shearmixing zones may discharge into 2 or 3 reactor vessels which areconnected in series. Where the reactor system comprises 2 or 3 reactorvessels, the high shear mixing zones may be divided substantiallyequally between the reactor vessels in the series. However, it is alsoenvisaged that high shear mixing zones may be unequally distributedbetween the reactor vessels in the series. Where the reactor systemcomprises a plurality of high shear mixing zones, the cooling tubesand/or cooling coils and/or cooling plates of the internal heatexchanger are substantially absent from the “blast” or “injection” zonesof each of the high shear mixing zones.

The high shear mixing zone(s) may comprise any device suitable forintensive mixing or dispersing of a gaseous stream in a suspension ofsolids in a liquid medium, for example, a rotor-stator device, aninjector-mixing nozzle or a high shear pumping means such as a propelleror paddle having high shear blades.

Preferably, the high shear mixing zone(s) is an injector mixingnozzle(s). Preferably the injector mixing nozzle(s) is at or near thetop of the reactor vessel and projects downwardly into the reactorvessel i.e. is a downshot nozzle. However, it is envisaged that theinjector mixing nozzle(s) may be at or near the bottom of the reactorvessel and projects upwardly into the reactor vessel i.e. is an upshotnozzle. The nozzle may also be angled, preferably at an angle of no morethan 25°, preferably no more than 10°, more preferably, no more than 5°relative to the longitudinal axis of the reactor vessel. Suitably, aplurality of injector mixing nozzles are spaced apart in the reactorvessel, preferably at or near the top or bottom of the reactor vessel sothat there is substantially no overlap between the blast zones of thenozzles. It is also envisaged that the reactor vessel may be providedwith additional nozzles (injector mixing nozzles, liquid only nozzles orgas only nozzles) which may be used to create turbulence in anyquiescent regions of the reactor vessel thereby avoiding sedimentationof the particulate catalyst.

The outlet of the nozzle(s) may be tapered outwardly so that the spraywhich exits the nozzle (suspension having gas bubbles and/or irregularlyshaped gas voids dispersed therein) diverges outwardly, preferably, atan angle of less than 60°, more preferably at an angle of less than 40°,most preferably at an angle of less than 30° relative to the initialdirection of discharge of the spray, for example, diverges outwardlyrelative to the longitudinal axis of the reactor vessel.

The injector-mixing nozzle(s) can advantageously be executed as aventuri tube (c.f. “Chemical Engineers' Handbook” by J. H. Perry, 3^(rd)edition (1953), p.1285, FIG. 61), preferably an injector-mixer (c.f.“Chemical Engineers' Handbook” by J H Perry, 3^(rd) edition (1953), p1203, FIG. 2 and “Chemical Engineers' Handbook” by R H Perry and C HChilton 5^(th) edition (1973) p 6–15, FIGS. 6–31) or most preferably asliquid-jet ejector (c.f. “Unit Operations” by G G Brown et al , 4^(th)edition (1953), p.194, FIG. 210). Where the injector-mixing nozzle(s) isa venturi tube, the constriction within the tube generally has adiameter of from 2.5 to 50 cm, preferably 5 to 25 cm. Alternatively, theinjector-mixing nozzle(s) may be executed as a “gas blast” or “gasassist” nozzle where gas expansion is used to drive the nozzle (c.f.“Atomisation and Sprays” by Arthur H Lefebvre, Hemisphere PublishingCorporation, 1989). Where the injector-mixing nozzle(s) is executed as a“gas blast” or “gas assist” nozzle, the suspension of catalyst is fed tothe nozzle at a sufficiently high pressure to allow the suspension topass through the nozzle while the gaseous stream comprising synthesisgas is fed to the nozzle at a sufficiently high pressure to achieve highshear mixing within the nozzle.

The high shear mixing zone(s) may also comprise an open-ended conduithaving a venturi plate located therein. Preferably, the venturi plate islocated close to the open end of the conduit, for example, within 0.5meters of the open end of the conduit. Suspension is fed down theconduit to the venturi plate at a sufficiently high pressure to passthrough apertures in the plate while a gaseous stream comprisingsynthesis gas is drawn into the conduit through at least one opening,preferably 2 to 50 openings, in the wall of the conduit. Preferably, theopening(s) is located immediately downstream of the venturi plate.Suitably, the venturi plate has between 2 to 50 apertures. Preferably,the apertures are circular having diameters in the range of 1 mm to 100mm. Suspension having gas bubbles and/or irregularly shaped gas voidsdispersed therein is then discharged into the reactor vessel though theopen end of the conduit. Suitably, the open end of the conduit istapered outwardly, preferably at an angle of 5 to 30°, preferably 10 to25° relative to the longitudinal axis of the conduit. Conveniently, theexternal conduit may recycle the suspension recycle stream to theopen-ended conduit.

Where the high shear mixing zone(s) is executed as a venturi nozzle(s)(either as a venturi tube or as a venturi plate), the pressure drop ofthe suspension over the venturi nozzle(s) is typically in the range offrom 1 to 40 bar, preferably 2 to 15 bar, more preferably 3 to 7 bar,most preferably 3 to 4 bar. Preferably, the ratio of the volume of gas(Q_(g)) to the volume of liquid (Q_(l)) passing through the venturinozzle(s) is in the range 0.5:1 to 10:1, more preferably 1:1 to 5:1,most preferably 1:1 to 2.5:1, for example, 1:1 to 1.5:1 (where the ratioof the volume of gas (Q_(g)) to the volume of liquid (Q_(l)) isdetermined at the desired reaction temperature and pressure).

Where the high shear mixing zone(s) is executed as a gas blast or gasassist nozzle(s), the pressure drop of gas over the nozzle(s) ispreferably in the range 3 to 100 bar and the pressure drop of suspensionover the nozzle(s) is preferably in the range of from 1 to 40 bar,preferably 4 to 15, most preferably 4 to 7. Preferably, the ratio of thevolume of gas (Q_(g)) to the volume of liquid (Q_(l)) passing throughthe gas blast or gas assist nozzle(s) is in the range 0.5:1 to 50:1,preferably 1:1 to 10:1 (where the ratio of the volume of gas (Q_(g)) tothe volume of liquid (Q_(l)) is determined at the desired reactiontemperature and pressure).

The high shear mixing zone(s) may also comprise an open-ended conduithaving a high shear pumping means positioned therein, for example apaddle or propeller having high shear blades. Suitably, the high shearpumping means is located close to the open end of the conduit forexample, within 1 metermeter, preferably within 0.5 meters of the openend of the conduit. A gaseous stream comprising synthesis gas isinjected into the open-ended conduit either immediately upstream ordownstream of the high shear pumping means, preferably, immediatelyupstream of the high shear pumping means. By immediately upstream ismeant that the gaseous stream is injected into the open-ended conduitless than 0.25 meters before the high shear pumping means. The gaseousstream may be injected into the open-ended conduit by means of asparger. Without wishing to be bound by any theory, the injected gaseousstream is broken down into gas bubbles and/or irregularly shaped gasvoids (hereinafter “gas voids) by the fluid shear imparted to thesuspension by the high shear pumping means and the resulting gas bubblesand/or gas voids become entrained in the suspension. The resultingsuspension containing entrained gas bubbles and/or gas voids is thendischarged into the reactor vessel through the open end of the conduit.Suitably, the open end of the conduit is tapered outwardly, preferablyat an angle of 5 to 25°, preferably 10 to 20° relative to thelongitudinal axis of the conduit. Conveniently, the external conduit mayrecycle the suspension recycle stream to the open-ended conduit.

Suitably, the volume of suspension present in the high shear mixingzone(s) is substantially less than the total volume of suspensionpresent in the reactor system, for example, less than 20%, preferablyless than 10% of the total volume of suspension present in the reactorsystem.

Preferably, the fluid shear imparted to the suspension in the high shearmixing zone(s) breaks down at least a portion of the gaseous reactantstream into gas bubbles having diameters in the range of from 1 μm to 10mm, preferably from 30 μm to 3000 μm, more preferably from 30 μm to 300μm which then become entrained in the suspension.

Without wishing to be bound by any theory, it is believed that theirregularly shaped gas voids are transient in that they are coalescingand fragmenting on a time scale of up to 500 ms, for example, over a 10to 500 ms time scale. The irregularly shaped gas voids have a wide sizedistribution with smaller gas voids having an average diameter of 1 to 2mm and larger gas voids having an average diameter of 10 to 15 mm.

Suitably, kinetic energy is dissipated to the suspension present in thehigh shear mixing zone(s) at a rate of at least 0.5 kW/m³ relative tothe total volume of suspension present in the system. Preferably, thekinetic energy dissipation rate in the high shear mixing zone is in therange of from 0.5 to 25 kW/m³, relative to the total volume ofsuspension present in the system, more preferably from 0.5 to 10 kW/m³,most preferably from 0.5 to 5 kW/m³, and in particular, from 0.5 to 2.5kW/m³.

In a preferred embodiment the process is carried out using at least oneinjector mixing nozzle, preferably a plurality of injector mixingnozzles. Very good mixing can be achieved when the injector-mixingnozzle(s) is situated at or near the top of the reactor vessel and thesuspension is removed from the reactor vessel at or near its bottom.Therefore the reactor vessel is preferably provided at or near its topwith at least one, preferably a plurality of injector-mixing nozzles andthe suspension recycle stream is preferably withdrawn from at or nearthe bottom of the reactor vessel. Preferably, the suspension recyclestream is, at least in part recycled via an external conduit, having aslurry pump positioned therein, to the top of the injector-mixingnozzle(s) through which it is then injected into the top of the reactorvessel. The gaseous stream comprising synthesis gas is introducedthrough one or more openings in the side wall of the injector-mixingnozzle(s). An internal heat exchanger comprising an array of coolingtubes and/or cooling coils and/or cooling plates is located in thereactor vessel in regions which are outside of the blast zone(s) of thenozzle(s) so that the spray from the nozzle(s) does not impinge on thearray. Without wishing to be bound by any theory it is believed that ifthe array extends into the blast zone(s) of the nozzle(s) that thisinterferes with the fluid dynamics in the reactor vessel and may alsoresult in coalescence of gas bubbles and erosion of the cooling tubesand/or cooling coils, and/or cooling plates of the array. Preferably, anexternal heat-exchanger is positioned on the external conduit to removeat least a portion of the exothermic heat of reaction from the system.

As discussed above, a gas cap (containing unconverted synthesis gas,gaseous higher hydrocarbons, vaporized higher hydrocarbons, vaporizedwater, any vaporized liquid coolant and any inert gases) may be presentin the top of reactor vessel above the level of the suspension.Suitably, the volume of the gas cap is not more than 40%, preferably notmore than 30% of the volume of the reactor vessel. The high shear mixingzone(s) may discharge into the reactor vessel either above or below thelevel of suspension in the reactor vessel. An advantage of the highshear mixing zone(s) discharging below the level of suspension is thatthis improves the contact between the synthesis gas and the suspensionin the reactor vessel.

A gaseous recycle stream may be withdrawn from the headspace of thereactor vessel and may be recycled to the high shear mixing zone(s). Thegaseous recycle stream is preferably cooled before being recycled to thehigh shear mixing zone(s), for example, by passing the gaseous recyclestream through a heat exchanger. The gaseous recycle stream may becooled to a temperature at which a two phase mixture of gas (comprisingunconverted synthesis gas, methane by-product, gaseous higherhydrocarbons and any inert gases, for example, nitrogen) and condensedliquid (water by-product, low boiling liquid higher hydrocarbons and anyliquid coolant) is formed. The condensed liquid may be separated fromthe gaseous recycle stream, for example, using a suitable gas-liquidseparation means (e.g. a hydrocyclone, demister, gravity separator) andat least a portion of the condensed liquid may be recycled to thereactor vessel or the high shear mixing zone(s), for example, with freshliquid coolant. Preferably, excess water by-product is removed from theseparated condensed liquids using a suitable separation means (e.g. adecanter), before recycling the condensed liquids to the reactor vesselor high shear mixing zone(s). It is envisaged that the heat exchangerand gas-liquid separation means may be combined within a single unit inorder to simplify recycling of the gaseous stream.

Fresh synthesis gas may be fed to the gaseous recycle stream, eitherupstream or downstream of the external exchanger. Where the synthesisgas has not been pre-cooled, the synthesis gas may be fed to the gaseousrecycle stream upstream of the heat exchanger. Preferably, the gaseousrecycle stream is recycled to the high shear mixing zone(s) via a bloweror compressor located downstream of the external heat exchanger.

Preferably, a purge stream is taken from the gaseous recycle stream toprevent the accumulation of gaseous by-products, for example, methane orcarbon dioxide, or any inert gases, for example, nitrogen in the system.If desired, any gaseous intermediate products may be separated from thepurge stream. Preferably, such gaseous intermediate products arerecycled to the reactor vessel where they may be converted to higherhydrocarbons.

In a further aspect of the present invention there is provided a reactorsystem for converting synthesis gas to higher hydrocarbons in thepresence of a suspension comprising a particulate Fischer-Tropschcatalyst suspended in a liquid medium, said reactor system comprising(i) a reactor vessel, (ii) at least one high shear mixing zone having anoutlet within the reactor vessel, (iii) an external conduit having afirst end in fluid communication with an outlet of the reactor vessel,said outlet being located at or near the bottom of the reactor vessel,and a second end in fluid communication with the high shear mixingzone(s), (iv) a first heat exchanger positioned within the reactorvessel and (v) a second heat exchanger positioned on the externalconduit characterized in that the heat exchange surfaces of the firstheat exchanger are located within regions of the reactor which areoutside of the blast zone(s) of the high shear mixing zone(s).

Preferred features of the reactor system are as described above inrelation to the process of the present invention.

Preferably, the ratio of hydrogen to carbon monoxide in the synthesisgas used in the process of the present invention is in the range of from20:1 to 0.1:1, especially 5:1 to 1:1 by volume, typically 2:1 by volumebased on the total amount of hydrogen and carbon monoxide introduced tothe system.

The synthesis gas may be prepared using any of the processes known inthe art including partial oxidation of hydrocarbons, steam reforming,gas heated reforming, microchannel reforming (as described in, forexample, U.S. Pat. No. 6,284,217 which is herein incorporated byreference), plasma reforming, autothermal reforming, and any combinationthereof. A discussion of these synthesis gas production technologies isprovided in “Hydrocarbon Processing” V78, N.4, 87–90, 92–93 (April 1999)and “Petrole et Techniques”, N. 415, 86–93 (July–August 1998). It isalso envisaged that the synthesis gas may be obtained by catalyticpartial oxidation of hydrocarbons in a microstructured reactor asexemplified in “IMRET 3: Proceedings of the Third InternationalConference on Microreaction Technology”, Editor W Ehrfeld, SpringerVerlag, 1999, pages 187–196. Alternatively, the synthesis gas may beobtained by short contact time catalytic partial oxidation ofhydrocarbonaceous feedstocks as described in EP 0303438. Preferably, thesynthesis gas is obtained via a “Compact Reformer” process as describedin “Hydrocarbon Engineering”, 2000, 5, (5), 67–69; “HydrocarbonProcessing”, 79/9, 34 (September 2000); “Today's Refinery”, 15/8, 9(August 2000); WO 99/02254; and WO 200023689.

Preferably, the higher hydrocarbons produced in the process of thepresent invention comprise a mixture of hydrocarbons having a chainlength of greater than 2 carbon atoms, typically greater than 5 carbonatoms. Suitably, the higher hydrocarbons comprise a mixture ofhydrocarbons having chain lengths of from 5 to about 90 carbon atoms.Preferably, a major amount, for example, greater than 60% by weight, ofthe higher hydrocarbons have chain lengths of from 5 to 30 carbon atoms.Suitably, the liquid medium comprises one or more higher hydrocarbonswhich are liquid under the process conditions.

The catalyst which may be employed in the process of the presentinvention is any catalyst known to be active in Fischer-Tropschsynthesis. For example, Group VIII metals whether supported orunsupported are known Fischer-Tropsch catalysts. Of these iron, cobaltand ruthenium are preferred, particularly iron and cobalt, mostparticularly cobalt.

A preferred catalyst is supported on a carbon based support, forexample, graphite or an inorganic oxide support, preferably an inorganicrefractory oxide support. Preferred supports include silica, alumina,silica-alumina, the Group IVB oxides, titania (primarily in the rutileform) and most preferably zinc oxide. The supports generally have asurface area of less than about 100 m²/g, preferably less than 50 m²/g,more preferably less than 25 m²/g, for example, about 5 m²/g.

The catalytic metal is present in catalytically active amounts usuallyabout 1–100 wt %, the upper limit being attained in the case ofunsupported metal based catalysts, preferably 2–40 wt %. Promoters maybe added to the catalyst and are well known in the Fischer-Tropschcatalyst art. Promoters can include ruthenium, platinum or palladium(when not the primary catalyst metal), aluminium, rhenium, hafnium,cerium, lanthanum and zirconium, and are usually present in amounts lessthan the primary catalytic metal (except for ruthenium which may bepresent in coequal amounts), but the promoter:metal ratio should be atleast 1:10. Preferred promoters are rhenium and hafnium.

The catalyst may have a mean particle size in the range 5 to 500microns, preferably 10 to 250 microns, for example, in the range 10 to30 microns.

Preferably, the suspension of catalyst discharged into the reactorvessel comprises less than 50% wt of catalyst particles, more preferably10 to 40% wt of catalyst particles, most preferably 10 to 30% wt ofcatalyst particles.

The process of the invention is preferably carried out at a temperatureof 180–380° C., more preferably 180–280° C., most preferably, 190–240°C.

The process of the invention is preferably carried out at a pressure of5–50 bar, more preferably 15–35 bar, generally 20–30 bar.

The process of the present invention can be operated in batch orcontinuous mode, the latter being preferred.

Suitably, the gas hourly space velocity (GHSV) for a continuous processis in the range 100 to 40000 h⁻¹, more preferably 1000 to 30000 h⁻¹,most preferably 2000 to 15000 h⁻¹, for example 4000 to 10000 h⁻¹ atnormal temperature and pressure (NTP) based on the feed volume ofsynthesis gas at NTP.

In a continuous process, in order to achieve a sufficiently highproductivity, the suspension should be present in the reactor vessel fora certain period of time. It has been found that the average residencetime of the liquid phase (i.e. the liquid component of the suspension)in the reactor vessel is advantageously in the range from 15 minutes to50 hours, preferably 1 to 30 hours. Where two reactor vessels areoperated in series it is preferred that the average residence time ofthe liquid phase in the first reactor vessel is in the range of 15minutes to 50 hours, preferably 1 hour to 30 hours and the averageresidence time of the liquid phase in the second reactor vessel is inthe range of 30 minutes to 30 hours.

Suitably, the gas residence time in the high shear mixing zone(s) (forexample, the injector-mixing nozzle(s) is in the range 20 millisecondsto 2 seconds, preferably 50 to 250 milliseconds.

Suitably, the gas residence time in the reactor vessel is in the range10 to 240 seconds, preferably 20 to 90 seconds.

Suitably, the gas residence time in the external conduit is in the range10 to 180 seconds, preferably 25 to 60 seconds.

In a continuous process, product suspension is continuously removed fromthe system, preferably from the external conduit and is passed to aliquid-solid separation means, where liquid medium and liquid higherhydrocarbons are separated from the catalyst. Preferably, entrainedgases are separated from the product suspension either within theliquid-solid separation means or prior to introducing the productsuspension to the liquid-solid separation mean, for example in theexternal heat exchanger. Examples of suitable liquid-solid separationmeans include hydrocyclones, filters, gravity separators, T-pieceseparators and magnetic separators. Alternatively, the liquid medium andliquid higher hydrocarbons may be separated from the catalyst bydistillation. Preferably, there are two or more product withdrawal lines(for two or more product side streams), each line leading to a dedicatedliquid-solid separation means. This ensures continuous operation of theprocess by allowing one or more of the liquid-solid separation means tobe taken off-line for cleaning. The separated liquids are then passed toa product purification stage where water by-product and liquid mediumare removed from the liquid higher hydrocarbons. The purification stagemay be simplified by using one or more of the liquid higher hydrocarbonsproduced in the process of the present invention as the liquid medium inwhich case there is no requirement to separate the liquid medium fromthe liquid higher hydrocarbons. The catalyst may be recycled as aconcentrated slurry to the reactor vessel. Fresh catalyst may be addedeither to the recycled slurry or directly into the reactor vessel.

The liquid higher hydrocarbons from the purification stage may be fed toa hydrocracking stage, for example, a catalytic hydrocracking stagewhich employs a catalyst comprising a metal selected from the groupconsisting of cobalt, molybdenum, nickel and tungsten supported on asupport material such as alumina, silica-alumina or a zeolite.Preferably, the catalyst comprises cobalt/molybdenum ornickel/molybdenum supported on alumina or silica-alumina. Suitablehydrocracking catalysts include catalysts supplied by Akzo Nobel,Criterion, Chevron, or UOP.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to theaccompanying drawings, in which:

FIG. 1 illustrates a reactor system in accordance with the invention;

FIG. 2 illustrates a schematic plan view of the reactor vessel (1)showing the arrangement of the nozzles (2) and the cooling tubes (9);and

FIGS. 3A and 3B illustrate a cooling tube (9) and a finned cooling tube(13), and

FIG. 3C illustrates a cooling plate (14).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a reactor system according to the present inventioncomprising a reactor vessel (1) and a plurality of venturiinjector-mixing nozzles (2). A gas cap (3) is present in the upper partof the reactor vessel (1), the lower part of which contains a suspension(4) of particulate catalyst suspended in the liquid higher hydrocarbons.A dotted line (5) denotes the upper level of the suspension (4) in thereactor vessel (1). Suspension (4) is recycled to the venturiinjector-mixing nozzles (2) via a line (6) and dedicated branch lines(7). Through one or more openings in the side walls of the venturiinjector-mixing nozzles (2) a gaseous phase comprising synthesis gas isdrawn into the nozzles (1) from the gas cap (3). Fresh synthesis gas isintroduced into the gas cap (3) via a line (8).

The suspension is cooled within the reactor vessel (1) by means of aplurality of cooling tubes (9) positioned within the reactor vessel (1)below the upper level of the suspension (5) and which are locatedoutside of the blast zones of the nozzles (2).

Via a lower outlet opening of the injector-mixing nozzles (2) thesuspension having synthesis gas entrained therein is discharged into thereactor vessel (1) below the level (5) of the suspension (4).Unconverted gaseous reactants then separate into the gas cap (3).

Suspension (4) is withdrawn from the bottom of the vessel (1) and atleast a portion of the suspension is recycled to the injector-mixingnozzles (2) by means of pump (10) and the line (6). The suspensionpassing through line (6) is cooled by means of an external heatexchanger (11).

Via a line (12) a portion of the suspension (4) is withdrawn from thesystem and is passed to a liquid-solid separation stage (not shown).

FIG. 2 illustrates a schematic plan view of the reactor vessel (1)showing the arrangement of the nozzles (2) and the cooling tubes (9).

FIGS. 3A and 3B illustrate a cooling tube (9) and a finned cooling tube(13) while FIG. 3C illustrates a cooling plate (14).

EXAMPLES Example 1

It was calculated that for a 30,000 barrel a day commercial plant,between 500 and 550 MegaWatts of heat must be removed from the system tomaintain the temperature of the suspension within the reactor vessel atthe desired reaction temperature, ideally, isothermal. The amount ofheat generated will depend on the type of catalyst and the productdistribution.

Table 1 shows the relationship between the diameter of the constrictionof a venturi nozzle and the number of venturi nozzles which would berequired for a 30,000 barrel per day plant (where the lower value forthe number of nozzles corresponds to a Q_(g):Q_(l) ratio of 1:1.5 andthe higher value for the number of nozzles corresponds to a Q_(g):Q_(l)ratio of 2.5:1).

TABLE 1 Number of venturi nozzles for a 30,0000 bbl/d plant Diameter ofrestriction in Number of Venturi Pressure in Reactor Nozzle(inch)Nozzles Bbl/d/nozzle Vessel (bara) 2 345–207  87–145 20 3 153–93 195–326 20 4 86–52 348–579 20 4 65–39 463–772 30 5 56–33 539–898 20 542–25  718–1197 30 6 40–24  752–1253 20 6 30–18 1002–1670 30

Table 2 illustrates the % reduction in the rate at which suspension isrequired to be fed to an external heat exchange (to maintain the desiredreaction temperature in the reactor vessel) with increasing cooling ofthe suspension in the external heat exchanger. In the base case, theheat exchanger is operated with the suspension being cooled to atemperature 12° C. below that of the suspension in the reactor vessel.It was found that in order to maintain substantially isothermalconditions in the reactor vessel, the suspension must be passed to theheat exchanger at a rate of 51,000 m³ of suspension per hour.

TABLE 2 Percentage Reduction in suspension recycle % reduction insuspension recycle ΔT (° C.) — 12 20 15 40 20 52 25 60 30

However, the desired reduction in the rate of recycle to the externalheat exchanger cannot be achieved using only an external heat exchangerowing to the risk of quenching of the Fischer-Tropsch synthesis reactionand/or deactivation of the catalyst in the external heat exchanger unit.Furthermore, without the presence of the internal heat exchanger unit itwould not be possible to control adequately the temperature of thesuspension in the reaction vessel. It is therefore necessary to removeat least a portion of the exothermic heat of reaction from the system bymeans of an internal heat exchanger positioned within the suspension inthe reactor vessel.

TABLE 3 Heat removal from a 30,000 bbl/day plant % Heat % Heat removedby removed by make-up vaporizing % Heat removed by % Heat removed byfeeds (fresh liquid External Heat Internal Heat synthesis gas) coolantExchanger Exchanger 5 0 95 0 51,000 m³/h recycle 5 5 50 40 27,000 m³/hrecycle 10 5 50 30 27,000 m³/h recycle 10 10 30 50 15,500 m³/h recycleTable 3 illustrates how heat may be removed from a 30,000 bbl/dayFischer Tropsch plant via an internal heat exchanger, an external heatexchangers and by injecting a stream comprising a vaporizable liquidcoolant. The total heat production of the plant is 505 MegaWatts.

Example 2

A suspension of aluminium oxide catalyst (15% w/w) in 800 litres oftetradecene was charged to a reactor system comprising a tank reactorvessel (straight length 4500 mm, diameter 420 mm) and an externalconduit. A gas liquid separator, a first and a second three-phasecentrifugal pump were positioned on the external conduit (the firstcentrifugal pump (P1080) located upstream and the second centrifugalpump (P1280) located downstream of the gas liquid separator). A 24 mmventuri nozzle was located in the upper region of the tank reactorvessel. Tetradecene was used as a mimic for the Fischer Tropsch wax,having similar physical properties at 30° C. as Fischer Tropsch wax at atemperature of between 200–250° C. Nitrogen was used as the gas feed tothe venturi nozzle and suspension having gas bubbles and irregularlyshaped gas voids dispersed therein was discharged from the nozzle belowthe level of suspension in the tank reactor vessel. The reactor systemwas pressurised from 0 to 30 bar (depending on the experimentalconditions) using nitrogen and the suspension was pumped around theexternal conduit using the three-phase centrifugal pumps. The reactorsystem was then allowed to reach steady state.

A series of measurements was taken around the loop in the absence of anyinternal reactor cooling tubes, as detailed in Table 4 below. A seriesof cooling tubes were then introduced into the tank reactor vessel. Thecooling tubes had an outer diameter of 25.4 mm. The cooling tubes werearranged in the tank reactor vessel such that there were 6 equi-spacedtubes located on a 200 mm PCD (PCD=pitch circle diameter) and 12equi-spaced tubes on a 340 mm PCD. The series of measurements was thenrepeated with the cooling tubes in place. The results are shown in Table5 below.

TABLE 4 No cooling tubes System pressure 20 barg nitrogen; Suspensionrecirculation rate 30 tonnes/hr gas/liquid ratio no gas 1:1 P1080% 87 87P1080 rpm 1479 1479 Pressure drop across P1080 (bar) 3.01 3.01 P1080head (m) 35.59 35.59 P1280% 84 88 Pressure drop across P1280 bar 3.013.29 P1280 rpm 1428 1496 P 1280 head (m) 35.59 38.9 Pressure in gasliquid separator (bar) 19.93 20.01 Pressure in external conduitdownstream of P1280 22.94 23.3 (bar) Pressure drop in venturi nozzlesuction chamber (bar) 1.03 0.74 FV1092% (position of flow valve on gasrecycle line 0 32 from the reactor headspace to the venturi nozzle) Flowrate of gaseous recycle stream from reactor 0 833 headspace to theventuri nozzle (kg/hr) Flow rate of gaseous recycle stream from the gas0 0 liquid separator to the venturi nozzle (kg/hr) Pressure in the tankreactor vessel (bar) 19.94 20.03 Pressure drop of suspension acrossventuri nozzle 2.55 2.8 (bar) Suspension circulation rate (tonnes/hr) 3031

TABLE 5 Cooling tubes System pressure 20 barg nitrogen; Suspensionrecirculation rate 30 tonnes/hr gas/liquid ratio No gas 1:1 P1080% 90 90P1080 rpm 1530 1530 Pressure drop across P1080 bar 3.14 3.14 P1080 head(m) 37.12 37.12 P1280% 100 100 Pressure drop across P1280 bar 2.8 3.11P1280 rpm 1700 1700 P1280 head (m) 33.1 36.77 Pressure in gas liquidseparator (bar) 19.97 20.19 Pressure in external conduit downstream ofP1280 22.77 23.3 (bar) Pressure drop over venturi nozzle suction chamber0.87 0.51 (bar) FV1092% (Position of flow valve on gaseous recycle 0 32line from reactor head space to the venturi nozzle) Flow rate of gaseousrecycle stream from reactor 0 735 headspace to venturi nozzle (kg/hr)Flow rate of gaseous recycle stream from gas liquid 0 0 separator toventuri nozzle (kg/hr) Pressure in reactor vessel (bar) 20 20.33Pressure drop of suspension across nozzle (bar) 2.02 2.39 Suspensioncirculation rate (tonnes/hr) 27 29

1. A process for the conversion of synthesis gas to higher hydrocarbonsby contacting a gaseous stream comprising synthesis gas, at an elevatedtemperature and pressure, with a suspension comprising a particulateFischer-Tropsch catalyst suspended in a liquid medium, in a systemcomprising at least one high shear mixing zone and a reactor vesselwherein the process comprises: (a) passing the suspension and thegaseous stream through a high shear mixing zone wherein the gaseousstream is broken down into gas bubbles and/or irregularly shaped gasvoids; (b) discharging suspension having gas bubbles and/or irregularlyshaped gas voids dispersed therein from the high shear mixing zone intothe reactor vessel; (c) maintaining the temperature of the suspensiondischarged into the reactor vessel at the desired reaction temperatureby means of an internal heat exchanger positioned within the suspensionin the reactor vessel wherein at least 5% of the exothermic heat ofreaction is removed from the system by means of the internal heatexchanger.
 2. A process as claimed in claim 1 wherein at least 10% ofthe exothermic heat of reaction is removed from the system by means ofthe internal heat exchanger.
 3. A process as claimed in claim 2 whereinat least 20% of the exothermic heat of reaction is removed from thesystem by means of the internal heat exchanger.
 4. A process as claimedin claim 3 wherein between 30 to 50% of the exothermic heat of reactionis removed from the system by means of the internal heat exchanger.
 5. Aprocess as claimed in claim 1 wherein suspension is withdrawn from thereactor vessel and is at least in part recycled to the high shear mixingzone after being cooled by means of an external heat exchanger.
 6. Aprocess as claimed in claim 5 wherein between 20 to 55% of theexothermic heat of reaction is removed from the system in the externalheat exchanger.
 7. A process as claimed in claim 6 wherein between 30 to50% of the exothermic heat of reaction is removed from the system in theexternal heat exchanger.
 8. A process as claimed in claim 5 wherein thesuspension recycle stream is cooled by means of the external heatexchanger to a temperature not more than 30° C. below, the temperatureof the suspension in the reactor vessel.
 9. A process as claimed inclaim 5 wherein the suspension is withdrawn from the reactor vessel andis recycled to the high shear mixing zone by means of an externalconduit having a first end in communication with an outlet for thesuspension in the reactor vessel and a second end in communication withthe high shear mixing zone and the external heat exchanger is positionedon the external conduit.
 10. A process as claimed in claim 9 wherein theratio of the volume of the external conduit—which excludes the volume ofthe external heat exchanger—to the volume of the reactor vessel is inthe range of 0.005:1 to 0.2:1.
 11. A process as claimed in claim 9wherein the suspension is recycled through the external conduit at arate of between (n×10,000) m³/h to (n×50,000) m³/h for a (n×30,000)barrel/day commercial plant where n is a number in the range 0.25 to 10.12. A process as claimed in claim 1 wherein the reactor vesselapproximates to a continuous stirred tank reactor (CSTR) having a Pecletnumber of less than 3 where the Peclet (Pe) number is defined by theequation:Pe=U _(g) H/δ where U_(g) is the gas velocity (ms⁻¹), H is the height ofthe suspension in the reactor vessel (m), and δ is the dispersioncoefficient (m²s⁻¹).
 13. A process as claimed in claim 1 wherein a heatexchange liquid is fed to the internal heat exchanger at a temperaturewhich is at least 25° C. below the temperature of the suspension in thereactor vessel.
 14. A process as claimed in claim 13 wherein a heatexchange liquid is fed to the internal heat exchanger at a temperaturewhich is at least 50° C. below the temperature of the suspension in thereactor vessel.
 15. A process as claimed in claim 14 wherein a heatexchange liquid is fed to the internal heat exchanger at a temperaturewhich is at least 100° C. below the temperature of the suspension in thereactor vessel.
 16. A process as claimed in claim 13 wherein the heatexchange liquid fed to the internal heat exchanger is selected from thegroup consisting of water, a solution of an inorganic salt, molteninorganic salts, a high boiling point oil and liquid sodium.
 17. Aprocess as claimed in claim 1 wherein the internal heat exchangercomprises an array of cooling tubes and/or cooling coils and/or coolingplates.
 18. A process as claimed in claim 17 wherein the array isdivided into 50 to 500 independently operated banks of cooling tubesand/or cooling coils and/or cooling plates.
 19. A process as claimed inclaim 17 wherein the cooling tubes of the array are arranged with theirlongitudinal axes aligned with the longitudinal axis of the reactorvessel.
 20. A process as claimed in claim 17 wherein the cooling tubeshave an outer diameter in the range 2 to 5 cm and are spaced apart fromeach other or from the walls of the reactor vessel by 10 to 20 cm.
 21. Aprocess as claimed in claim 17 wherein the cooling tubes are absent froma blast zone of the high shear mixing zone.
 22. A process as claimed inclaim 17 wherein each cooling coil of the array is in the form of ahelix with the coil wound as if along a cylinder.
 23. A process asclaimed in claim 22 wherein the tubing of the cooling coils has an outerdiameter of between 2.5 cm and 10 cm and the cooling coils are spacedapart from each other or from the walls of the reactor vessel by 10 to20 cm.
 24. A process as claimed in claim 22 wherein the blast zone of ahigh shear mixing zone is arranged so as to lie within the cylinderdefined by the helix of the cooling coil and the diameter of thecylinder defined by the helix is at least 3 times the diameter of theoutlet of a high shear mixing zone.
 25. A process as claimed in claim 17wherein the heat exchanger comprises an array of concertinaed orcorrugated cooling plates having a breadth of from 2 to 10 cm and adistance across the folds of the concertinaed plates or between thepeaks and troughs of the corrugated plates of from 10 to 50 cm and thecooling plates are spaced apart from each other and from the walls ofthe reactor vessel by at least 10 cm.
 26. A process as claimed in claim17 wherein the cooling tubes, cooling coils or cooling plates arefinned.
 27. A process as claimed in claim 17 wherein the cooling tubes,cooling coils or cooling plates lie below the level of suspension in thereactor vessel and extend through up to 80% of the height of thesuspension in the reactor vessel.
 28. A process as claimed in claim 1wherein at least 5% of the exothermic heat of reaction is removed fromthe system by introducing a vaporizable liquid coolant to the reactorvessel and/or the high shear mixing zone at a temperature which is atleast 25° C. below the temperature of the suspension in the reactorvessel.
 29. A process as claimed in claim 28 wherein the vaporizableliquid coolant is selected from the group consisting of aliphatichydrocarbons having from 5 to 10 carbon atoms, alcohols having from 1 to4 carbon atoms, ethers, tetrahydrofuran and water.
 30. A process asclaimed in claim 1 wherein the reactor system comprises 10 to 400 highshear mixing zones and the high shear mixing zones discharge into asingle reactor vessel or into 2 or 3 reactor vessels connected inseries.
 31. A process as claimed in claim 1 wherein the high shearmixing zone comprise (a) an injector-mixing nozzle(s) or (b) anopen-ended conduit(s) having a high shear pumping means positionedtherein and a gas sparger located immediately upstream or downstream ofthe high shear pumping means.
 32. A process as claimed in claim 31wherein the injector-mixing nozzle(s) is executed as a venturi nozzle(s)or as gas blast nozzle(s).
 33. A process as claimed in claim 32 whereinthe pressure drop of the suspension over the venturi nozzle(s) is in therange of from 2 to 15 bar and the ratio of the volume of gas (Q_(g)) tothe volume of liquid (Q_(l)) passing through the venturi nozzle(s) is inthe range 1:1 to 5:1 measured at the conditions of elevated temperatureand pressure.
 34. A process as claimed in claim 32 wherein the pressuredrop of gas over the gas blast nozzle(s) is in the range 3 to 100 bar,the pressure drop of the suspension over the gas blast nozzle(s) is inthe range of from 4 to 15 bar and the ratio of the volume of gas (Q_(g))to volume of liquid (Q_(l)) passing through the gas blast nozzle(s) isin the range 1:1 to 10:1 measured at the conditions of elevatedtemperature and pressure.
 35. A process as claimed in claim 32 whereinthe injector mixing nozzle(s) are located at or near the top of thereactor vessel and project downwardly into the reactor vessel.
 36. Aprocess as claimed in claim 35 wherein the injector mixing nozzle(s) areangled at an angle of no more than 5° relative to the longitudinal axisof the reactor vessel.
 37. A process as claimed in claim 32 wherein theoutlet of the nozzle(s) are tapered outwardly so that the spray whichexits the nozzle diverges outwardly at an angle of less than 30°relative to the initial direction of discharge of the spray.
 38. Aprocess as claimed in claim 1 wherein the fluid shear imparted to thesuspension in the high shear mixing zone breaks down at least a portionof the gaseous stream comprising synthesis gas into gas bubbles havingdiameters in the range of from 30 μm to 3000 μm which bubbles becomeentrained in the suspension.
 39. A process as claimed in claim 1 whereinthe irregularly shaped gas voids dispersed in the suspension dischargedinto the reactor vessel either coalesce to form larger gas voids orfragment to form smaller gas voids with the gas voids having an averageduration of up to 500 ms.
 40. A process as claimed in claim 1 whereinkinetic energy is dissipated to the suspension present in the high shearmixing zone at a rate of from 0.5 to 25 kW/m³, relative to the totalvolume of suspension present in the system.
 41. A process as claimed inclaim 1 wherein the process is operated in continuous mode and theaverage residence time of the liquid component of the suspension in thereactor vessel is in the range from 15 minutes to 50 hours.
 42. Aprocess as claimed in claim 5 wherein up to 50% by volume of thehydrogen component of the synthesis and/or up to 50% by volume of thecarbon monoxide component of the synthesis gas is introduced into thesuspension recycle stream.
 43. A process as claimed in claim 29 whereinthe alcohol is selected from methanol and ethanol, and the ether isdimethyl ether.